Operation of gas turbine engines is well known along with use of nozzle guide vanes in order to guide gas flow within the engine for improved performance. The nozzle guide vanes are essentially located in the flow path between the casing and core of the engine. Dependent upon position these guide vanes are clearly subjected to high temperatures and so require cooling. In order to provide that cooling it will be understood that cooling passages will be provided particularly at the surfaces of the guide vane through which coolant air passes in order to cool the vane in operation.
The aerofoil thickness of a nozzle guide vane relates directly to turbine efficiency within a gas turbine engine. The speed of flow over the suction surface can attain supersonic speeds. Reducing aerofoil thickness can reduce flow blockage which in turn reduces flow speed and increases engine performance. In such circumstances, reduced aerofoil thickness can have advantages but it will also be understood that generally service pipes in order to provide coolant air and for lubricants supplied to and from an engine core hub will also be required. Accommodation of such service pipes is generally more difficult with a reduced aerofoil thickness.
Existing designs for coupling service pipes provide a pipe which passes through the guide vane from a hub. An end fitting is secured to the pipe with a top hat element then secured over the end fitting for coupling to external supply passages, and as indicated previously possibly for coolant air or oil. It will be appreciated that the top hat and end fitting essentially provide three functions. The primary function is to provide a robust mechanical connection for an external union joint e.g. nut and bolt assembly to be fastened onto in order to create the coupling between them. A second function is to provide a spherical seal to prevent leakage and so release of high pressure air or lubricant within the casing escaping to atmosphere outside of the casing. A final function is to provide an anti rotation feature which prevents pipe damage on tightening the threaded conical union joint as described above as the primary function. It will also be understood that the coupling can articulate around the centre of the spherical seal and also slide in a radial direction along the axis of the pipe to accommodate for thermal growth during engine operational thermal cycling.
These prior arrangements provide an anti-rotation function by virtue of a symmetrical arrangement. In short the top hat connector will fully encircle the anti-rotation lug features providing reaction points on two of the faces when tightening and the other two faces when untightening. Such an approach is acceptable where there is ample clearance between the vane end incorporating the coupling and the casing as the coupling can be situated externally of the vane platform. However, increasingly engine geometry provides much reduced space between the vane and the casing. In such circumstances an outboard top hat will result in a clash between the top hat part of the coupling and the vane. Previous solutions to this problem have been to increase the spacing by moving the casing further outboard and so increasing the clearance but with the detrimental effect of increasing overall engine weight due to a larger than otherwise needed casing. The other alternative is to provide a wider guide vane to accept the symmetrical top hat, but as indicated previously such widening of the guide vane will have a major detrimental effect upon turbine efficiency in terms of gas flow and therefore engine performance.
It is important that there is no clash between the top hat member and the vane as this may create distortion and straining of the pipe with potential failure of the seal, etc whilst as indicated accommodation of the top hat member will require either increasing the casing size adding to weight or increasing the width of the vane reducing aerodynamic efficiency.